The invention relates to a block copolymer or graft copolymer with weight-average molar mass Mw of at least 100 000 g/mol, comprising
a) at least one block S composed of from 95 to 100% by weight of vinylaromatic monomers and from 0 to 5% by weight of dienes, and
b) at least one copolymer block (S/B)A composed of from 63 to 80% by weight of vinylaromatic monomers and from 20 to 37% by weight of dienes, with glass transition temperature TgA in the range from 5 to 30° C.,
where the proportion by weight of the entirety of all of the blocks S is in the range from 50 to 70% by weight, based on the block copolymer or graft copolymer, and also mixtures thereof, and their use.
U.S. Pat. No. 3,639,517 describes star-shaped branched styrene-butadiene block copolymers having from 75 to 95 percent by weight of terminal blocks composed of vinylaromatic monomers, and from 5 to 30 percent by weight of elastomeric blocks mainly composed of conjugated diene units. They can be blended with standard polystyrene to give highly transparent mixtures. With increasing proportion of polystyrene, modulus of elasticity rises, with attendant losses in toughness. Mixtures using as little as 40 percent by weight of polystyrene are too brittle for most applications. If acceptable ductility is to be retained, the possible admixture of polystyrene is mostly only 20, up to a maximum of 30, percent by weight.
Star-shaped block copolymers having 40% by weight of hard blocks composed of vinylaromatic monomers, and soft blocks having random structure composed of vinylaromatic monomers and dienes, are described in WO 00/58380. They are blended with standard polystyrene in order to increase stiffness, whereupon transparency falls. Even with 60 percent by weight of polystyrene, they continue to give ductile mixtures. The disadvantage of these blends is the clearly visible haze, which is unacceptable for more demanding applications and thicker components.
WO 2006/074819 describes mixtures of from 5 to 50% by weight of a block copolymer A, which comprises one or more copolymer blocks (B/S)A in each case composed of from 65 to 95% by weight of vinylaromatic monomers and from 35 to 5% by weight of dienes, with glass transition temperature TgA in the range from 40° to 90° C., and from 95 to 50% by weight of a block copolymer B which comprises at least one hard block S composed of vinylaromatic monomers, and one or more copolymer blocks (B/S)B in each case composed of from 20 to 60% by weight of vinylaromatic monomers and from 80 to 40% by weight of dienes, with glass transition temperature TgB in the range from −70° to 0° C., for the production of shrink foils. The stiffness of the mixtures is in the range from 700 to a maximum of 1300 MPa.
EP-A 1 669 407 discloses mixtures composed of linear block copolymers composed of vinylaromatic monomers and dienes of the structure (I) S1-B1-S2 and (II) B2-S3. The blocks B1 and B2 can be composed exclusively of dienes, or of dienes and vinylaromatic monomers. The ratio by weight of vinylaromatic monomer to diene for the blocks B1 and B2 is preferably in the range from 0.3 to 1.5.
PCT/EP2008/061635, as yet unpublished, describes transparent, tough and stiff molding compositions based on styrene-butadiene block copolymer mixtures which can comprise, inter alia, from 0 to 30% by weight of a block copolymer which comprises at least one copolymer block (B/S)A in each case composed of from 65 to 95% by weight of vinylaromatic monomers and from 35 to 5% by weight of dienes, with glass transition temperature TgA in the range from 40 to 90° C., and at least one copolymer block (B/S)B in each case composed of from 1 to 60% by weight of vinylaromatic monomers and from 99 to 40% by weight of dienes, with glass transition temperature TgB in the range from −100 to 0° C.
Any desired modulus of elasticity extending to above 3000 MPa can be obtained via blending of conventional styrene-butadiene block copolymers, such as Styrolux®, with polystyrene, as a function of mixing ratio. However, experience has shown that no useful ductility is retained when the modulus of elasticity is above 1900 MPa. The mechanical behavior of the mixtures is then similar to that of polystyrene itself, and they then have no advantages over the latter.
Blister packs, thermoformed containers and pots, and packaging materials for electronic components, for example extruded hollow profiles used as transport tubes for integrated circuits, require a combination of high stiffness and ductility and good transparency, while dependably exceeding the required yield stress value. These are applications for which polystyrene and its mixtures with styrene-butadiene block copolymers have hitherto had no, or only limited, suitability. The market has hitherto been covered by polyvinyl chloride (PVC), and to some extent by polyethylene terephthalates (PET), or very expensive specialty polymers.